SPOILER ALERT!!!
I like the way this movie went, it's a good portray of the consequences of trauma.
Cheryl, well, as weird as it might seem to some people, there are lot like her. Maybe not to the point she's reached, but we can see the emotional scar she got from her past and what she had to endure, or at least what she says she had to endure because at this point, we don't even know if she really said the truth or was highly hyperbolic about it and she was just not right in her mind from the start.
If she was born with mental issue, we can theorize that it might have rubbed off on her 2 first children, on top of the life they've had, it doesn't help.
It also interesting to see her two youngest being intrigued and seeking who they are. In fact, we can see the paradox in what Cheryl says; at some point she explain that's in normal for someone to seek who they are, learn about where they come from etc... Which is exactly what she denies to her children by putting everything "Black off limits". I think that this lack of knowledge about themselves played a big role into the development of the story, because they saw the two oldest and instantly "connected", seeing a way to find themselves maybe.
As for the two oldest.... Well, we don't know what they come from truly. We just know that their mother and father abandoned them, that their aunt did to and they've been given away, but we don't know if there's more to it; this type of thing can be the best or the worst thing to happen to someone depending on where they end up and how they're treated, but we can assume that nothing went well.
We could called them crazy, including the mom, but I think there's something deep in this movie, something that people judge easily without ever questioning themselves about how you can place judgment upon someone you don't know. I think the three of them were totally lost, the mother trying to create a new self, and the children trying to find themselves, and get what they think they deserved and have been deprived of.
About the end, remarkable.
I have two take on it, the first being the mother staying true to herself. She wants a life for herself, self projecting when they have the talk in the living room, trying to make sure she will be able to maintain the control she's acquired all these years. She left at the beginning, and left at the end. She is who she is and wants what she wants, nothin and no one will take it from her.
My second take, the oldest are doing this to reach the mother, they don't care much about the whole family except the part where they are jealous.
As Cheryl leaves, she makes a point: It's not you, it's me. I did it to you and do it again them, which, as horrible as one can take it, can give some closure to the oldest and make them realize that it's just who she is, but also, that could save the children. As she leaves, with the father who died, the bike sounds means that the oldest have nothing else to take from the youngest, nothing to be envious about, in fact, the youngest might be even more perturbed as they are at an age where you take those things to a whole other level. So i could see it as a blessing of saving their life, but a curse because if they don't overcome what happened, it will not be a pleasant one.
I think that the missing information gives life to this story by allowing us to think, and wonder, to discuss, and share our thoughts, see who got what and debate.
That's it for me, I enjoyed the movie and recommend it.